produces the stack (ab]{\displaystyle \left({a \atop b}\right]} where it has delimiter ( on the left and ] on the right. The delimiters are treated as if \left precedes the left delimiter and \right precedes the right delimiter, and hence the delimiters scale based on the size of the contained text. A null delimiter can be used by putting a period . in for one of the delimiters. Hence \atop can be replicated as

\atop = \atopwithdelims..

One important use of \atopwithdelims is in the definition of the Plain TeX command \choose